Re: [LARTC] DNAT PREROUTING issue with IPTABLES

[Aleksander Kamenik <aleksander@krediidiinfo.ee>]

Indunil Jayasooriya wrote:

>    SECOND Firewall's default route (gateway) is NOT the FIRST firewall.
>  BOTH firewall's default route (gateway) is the router given by our ISP.

Ok, so you understand your problem now?

Assuming the packet arrives at 1.2.3.4 from random external ip (eg. 
5.5.5.5), is successfully dnat+rerouted to 2.3.4.5, there again 
dnat+reroute to 192.168.x.x. Arrives at smtp server and smtp server 
sends a reply to the original sender 5.5.5.5. It does that via it's 
default gateway which I assume is 2.3.4.5. 2.3.4.5 sends it via your 
ISP's gateway with it's own address of 2.3.4.5 to 5.5.5.5.

But 5.5.5.5 sent the packet 1.2.3.4, not 2.3.4.5, so it discards it.

And that's exactly what Riccardo said when I read his mail now.

The first problem though is that I'm not sure the dnat form 1.2.3.4 to 
2.3.4.5 works, the packet would have to leave via the same interface it 
came. Maybe this works, I've never tried that. Make sure packets arrive 
on the smtp box with tcpdump.

As for the solution, one way would be to SNAT the connection at FW1, but 
this wwould cause the smtp box to see as if all the incoming connections 
are from 1.2.3.4 and not their real IP's (5.5.5.5).

Actually you should set up custom routing at 1.2.3.4 and not DNAT. You'd 
have to mark the packets and then send them to the 2.3.4.5 fw via a 
custom route. I'm not sure I could help you with that, never done any 
advanced routing.

-- 
Aleksander Kamenik
system administrator
+372 6659 649
aleksander@krediidiinfo.ee

Krediidiinfo AS
http://www.krediidiinfo.ee/
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